1. Field of the Invention
The present invention relates to a method of manufacturing a metal gasket, and more particularly to a method of manufacturing a metal gasket having a metallic base plate and coating layers deposited thereon, and disposed between a cylinder head and a cylinder block of an internal combustion engine.
2. Description of the Related Art
It has been well known that a gasket is disposed between connecting members so as to seal a clearance therebetween. For instance, between a cylinder head and a cylinder block of an internal combustion engine, a cylinder head gasket is disposed. With respect to this gasket, a metal gasket formed of a metallic plate has been noted, nowadays. For example, Japanese Utility model Laid-Open Publication No. 61-41960 discloses a metal gasket which has a metallic plate with a vulcanized first coating layer deposited thereon and a non-vulcanized second coating layer deposited on the first coating layer to provide two coating layers. Also, Japanese Utility model Laid-Open Publication No. 1-146064 discloses a metal gasket having a base plate, on which an unvulcanized coating layer including a vulcanizing agent is deposited. In the latter publication, a soft coating layer which contains no vulcanizing agent is defined as a non-vulcanized coating layer, and the one which contains the vulcanizing agent but which has not been vulcanized, is defined as an unvulcanized coating layer. Also, in the Publication No. 61-41960, terminologies of "vulcanized" and "non-vulcanized" are defined in the same meaning as defined above.
In Japanese Patent Laid-Open Publication No. 2-227217, it has been pointed out that the connecting force between the vulcanized rubber layer and non-vulcanized rubber layer of the metal gasket disclosed in the Publication No. 61-41960 is weak so that the layers might be separated, and that the manufacturing process is complicated and takes a long time, and therefore a method to solve the foregoing problems has been proposed. That is the method of forming a vulcanizing agent layer containing a vulcanizing agent on a surface of a base material, then forming further a non-vulcanized rubber layer on the vulcanizing agent layer, and vulcanizing the non-vulcanized rubber layer to form a vulcanized rubber layer with a vulcanized rate which is decreased from the surface of the base material to the surface of the rubber layer.
In this publication No. 1-146064, it has been pointed out that the metal gasket having the vulcanized first coating layer and the non-vulcanized second coating layer deposited thereon as disclosed in the publication No. 61-41960 has such a problem that the non-vulcanized layer will be flown out to impair a sealing property, and therefore it has been proposed that the unvulcanized coating layer is deposited to satisfy the requirements of the sealing property and durability at the same time. However, the unvulcanized coating layer will be heated and hardened gradually after the gasket was clamped between the connecting members, so that the layer may not obtain a sufficient sealing effect depending upon the vulcanizing agent contained in the unvulcanized coating layer.
As for the vulcanizing agent, there is known that of a cold type which accelerates a vulcanization at an ordinary temperature or room temperature, and also that of a hot type which accelerates the vulcanization at a higher temperature than the ordinary temperature, for example. If the vulcanizing agent contained in the unvulcanized coating layer is of the former type, i.e., the cold type vulcanizing agent, the unvulcanized layer will be vulcanized in an ordinary storing condition of a metal gasket in accordance with a lapse of time after it was manufactured, so that the unvulcanized coating layer will be hardened to impair its sealing effect. Therefore, it is not appropriate to store the metal gasket for a long period of time. As a countermeasure, it will be necessary to store it in a low temperature room. Whereas, if the hot type vulcanizing agent is employed, the above-described problem about the unvulcanized coating layer will be solved, but it will be necessary to heat the layer at a temperature which is higher than the temperature at which the layer having the cold type vulcanizing agent is vulcanized, so that a large energy will be consumed to raise a manufacturing cost.
On the other hand, according to a method recited in the publication 2-227217, the vulcanizing agent in the vulcanizing agent layer is spreading and combined to form the vulcanized rubber layer, so that the sealing property and durability will be obtained and the manufacturing process will be simplified. However, the spreading of the vulcanizing agent into the non-vulcanized rubber layer is dependent on various factors such as the thickness of the vulcanizing agent layer, the temperature or the like, so that it is not necessarily easy to make uniform the spreading in each metal gasket. Consequently, a dispersion in a distribution within the vulcanized rubber layer will be caused among the products to make it difficult to ensure a certain quality of each product.